Conventionally, an image reading apparatus is provided with an optical system including a single fixed-focus lens. The magnifying power of the optical system may be varied by changing the length of its optical path and moving the lens. The magnifying power of an optical system having a zoom lens may be varied by changing the focal length of the zoom lens.
However, the optical system which has a single fixed-focus lens cannot set its magnifying power to a large value, and requires complex mechanisms for changing the length of the optical path and moving the lens. The optical system with a zoom lens is disadvantageous in that the lens performance is lowered when the magnifying power is set to a large value. Because of various problems that are posed by a large magnifying power value, both of these optical systems require improvements.
Various mechanisms have heretofore been devised and implemented for transporting a movable base between first and second positions and accurately positioning and holding the movable base at these positions. To accomplish this, it is preferable to have the movable base contact the first and second positioning stops which define these positions.
If the movable base is kept in contact with the first and second positioning stops by drive forces produced by a motor, then the motor will have an excessive load, consume a large amount of electric energy, and become excessively heated. After the motor is de-energized, since no forces are applied to hold the movable base in the first or second position, the movable base is free to move, and does so when an apparatus, that incorporates this mechanism, is moved around.
If the motor is to be de-energized while the movable base contacts the first or second positioning stop, then a highly accurate sensor is needed to detect the present position of the movable base. Use of such a sensor results in an increased cost.
Previously, there has been devised and used a reflecting mirror assembly for moving a reflecting mirror between a first position, which is an upper position, and a second position, which is a lower position. To accurately position and hold the reflecting mirror in the upper and lower positions, it is preferable that the reflecting mirror contact the upper and lower positioning stops which define these positions.
If the reflecting mirror is kept in contact with the upper and lower positioning stops by drive forces produced by a motor, then the motor will have an excessive load, consume a large amount of electric energy, and become excessively heated. To avoid these difficulties, the reflecting mirror may be driven to an upper position by the motor, and then a coil spring may be used to hold the reflecting mirror in the upper position, thus requiring that no force be applied from the motor to the reflecting mirror. Since, however, the reflecting mirror is held in the upper position only by the coil spring, the reflecting mirror may be forcibly moved from the upper position to a lower position against the bias of the coil spring, when shocks are imposed on the reflecting mirror, when it is in the upper position.